mid-15c., "person who kills a brother;" 1560s, "act of killing a brother," from Latin fratricida "brother-slayer," from frater "brother" (see brother) + cida "killer;" in the later use from cidum "a killing," both from caedere "to kill, to cut down" (see -cide). Among several Old English words for this were broðorbana "one who kills a brother;" broðorcwealm "act of killing a brother."